1. Field of the Invention
This invention relates to a brushless motor, or more in particular to a brushless motor using a semiconductor switching element, especially a transistor switching element.
2. Description of the Prior Art
A brushless motor is well known which comprises a rotor made of a magnetized permanent magnet and a stator provided around the rotor in spaced relationship therewith, so that the current in the stator windings is controlled by magnetically detecting the rotor position. In many of this type of small-sized brushless motor, a transistor is used as a switching means to control the current in the stator windings. As is well known, the transistor has an emitter, a collector and a base and controls the current between the emitter and collector by means of the current flowing in the base, except for the field effect transistor in which the current is controlled by a gate voltage. The control range of these transistors including the thyristor, however, depends on the magnitude of voltage and it is difficult to accomplish the switching operation successfully in an electrical circuit involving a high voltage. It is possible to widen the control range by the use of a transistor with a high breakdown voltage, but it has the disadvantage of high cost. Further, it is technically difficult to provide a high breakdown voltage in an integrated circuit construction of the control circuit network.
On the other hand, a voltage is often induced by the inductance in the winding when the motor winding current is controlled, especially when the winding current is cyclically cut off. The magnitude of the induced voltage, the generation of which is only for a short duration, is proportional to the speed with which the current is cut off, the magnitude of the current cut off and the magnitude of the inductance. Therefore, if a transistor with a comparatively low breakdown voltage is to be used, some measure must be taken to prevent the generation of the induced voltage.
Also, it is necessary to review such a preventive measure when the voltage induced in the stator windings by the rotation of the rotor is utilized in automatically regulating the motor at a constant speed.